This invention relates to the detection of the maximum instantaneous value of analog signals from multiple sources such as is required in autofocus systems and the like, and in particular the invention relates to gating circuits for accomplishing that end.
The autofocus concept is based on the principle that as an imaging system comes into focus the peak video outputs from detector electronics increase in amplitude to a maximum value. To insure that the autofocus system works under all field conditions, the system must direct the maximum instantaneous video from any channel of a multi-channel scanning seeker and not add or subtract the channels video. Infrared imaging carousel systems have one or more video channels. Present systems typically have four video outputs.
The detection of the maximum instantaneous value of signals from multiple sources has, in the past, been accomplished by either diode OR gate circuits or by common emitter transistor OR gate circuits. These circuits, however, have been found to be deficient in various aspects. Diode OR gate circuits are not linear, have temperature dependent threshold voltage, require matched parts, and have an output impedance that is dependent upon load current. Common emitter transmitter OR gate circuits require matched parts, have a limited dynamic range, and have a voltage gain that is less than unity.
The present invention is directed toward providing an analog OR gate circuit that is an improvement over such circuits and that is not subject to the various deficiencies enumerated above.